Solid models on the shop floor

Designers use 3-D solids to better visualize parts. But how can these CAD images benefit the molding process? Custom injection molder Mission Plastics Inc. (Rancho Cucamonga, CA) offers an answer. Mission uses computer-generated solid models to guide manufacturing personnel on the best ways to mold parts.

The firm is ISO 9002-certified and specializes in products ranging from precision flashlight components to irrigation systems and building materials. Operators routinely work with engineering thermoplastics as well as PVC and CPVC, materials well-suited for making complex, durable parts but which require tight production tolerances. To maintain consistently high quality, workers must constantly be on the lookout for even slight defects in parts such as short shot areas, flash, distortion caused by thermal differences, and discoloration produced by resin contamination.

Engineers describe possible flaws and indicate areas where they are most likely to occur in what Mission Plastics and many others call visual standards. These documents typically contain diagrams and descriptive text and are then posted on the press.

Visual standards are also useful as a tool for evaluating customer-supplied molds by allowing engineers to assess such characteristics as wall thickness, gate locations, and areas where ribs are attached to sidewalls.

In the past, diagrams for the visual standards were isometric line drawings created with a 2-D drafting system. The drawings took hours to prepare and often left details open to interpretation. Moreover, different views needed to answer questions on the shop floor or resolve manufacturing problems once molding was under way required entirely new drawings to be generated, thus further delaying production operations.

"Isometrics were time-consuming to draw and ambiguous to interpret," explains Christopher Moralez, a manufacturing engineer at Mission Plastics. "In many cases, the diagrams didn't clearly communicate design geometry to operators and turned out to be a bottleneck in the shop."

To overcome these problems, Mission Plastics installed a Helix solid modeling system from Microcadam Inc. (Los Angeles) to quickly produce 3-D graphics. Visual Realty software from Micrografx Inc. (Atlanta) was also installed to produce color shaded hardcopies on the company's Hewlett-Packard DeskJet 680C inkjet printer. Text and notes are added with WordPerfect, and arrows pointing to appropriate areas on the models are put in with PaintBrush. The packages run on an IBM Pentium 100 PC with 64 Mb of RAM.

According to Moralez, its solid modeling system not only produces clearer, much more realistic diagrams than isometrics; it also saves considerable time. Moralez's experience indicates that solid models and isometrics take about the same time to develop initially: about 4 to 6 hours. "You lay down lines and try to imagine part shape in drawing isometrics," he says. "In solid modeling, you combine basic shapes with Boolean operations, or you can extrude, rotate, or sweep profile curves to define 3-D geometry."

"The savings come in when we have to generate new views," explains Moralez. "Isometrics must be done from scratch and take another several hours. But I can change Helix views in only 2 minutes by rotating and manipulating the solid model. Cutaways and sectional views are just as easily produced to show hidden details."

Moralez says he also finds solid models provide a more accurate representation of the complex surfaces found on an increasing number of parts the company molds. Helix includes advanced skinning functions for modeling double-curved shapes and has trimming, blending, rounding, filleting, chamfering, and offsetting operations for defining details.

Within Helix, these functions are driven by a Designbase solid modeling engine from Ricoh Corp. (San Jose, CA). The engine also maintains a history tree of the design operations displayed in a separate window, allowing users not only to maintain a record of operations but also to modify the design by editing the sequence of operations right on the history tree in a process called metamodeling. This capability is useful in jumping back to any point in the design history to quickly correct errors, explore design alternatives, and otherwise make changes, according to Moralez. The system also supports parametric features so that the design is automatically reconfigured when key dimensions are changed.

Another advantage Moralez found in using solid modeling is the ability to accurately calculate mass properties such as weight and volume in a matter of seconds. In the past, engineers would estimate using a series of representative cross sections, a time-consuming approach that produced only rough approximations and was particularly troublesome on parts with intricate details.

Solid modeling also fits into the company's plans to soon offer its customers rapid prototyping services to produce an actual part for evaluation purposes before molding operations begin. In fact, one of the reasons Mission Plastics selected Helix was for its ability to generate the STL (stereolithography protocol language) files used by rapid prototyping systems.

"Solid modeling lets us reduce turnaround times, maintain tight tolerances, and suggest design changes that customers might not have thought of initially to maximize strength, reduce materials, and lower costs," explains Ron Miller, operations manager at Mission Plastics. "In this respect, the technology is a key element in our corporate strategy of working in partnership with our customers to produce high-quality molded parts to meet their needs as quickly and economically as possible."